The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of images in a fast cardiac MRI acquisition.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When applied to cardiac imaging, for example, a complete scan from which a series of images showing the heart at different phases of its cycle or at different slice locations can be acquired in a single breath-hold.
There are two common techniques for acquiring cardiac MR images. The first is a prospectively gated, single-phase, multi-slice conventional spin echo sequence. In each cardiac cycle, data at different spatial locations are acquired with the same k-space phase encoding value. Images at the different spatial locations are then acquired at different temporal phase of the cardiac cycle. Since only one k-space line is acquired per cardiac trigger, a typical scan with a 128 k-space views in the phase encoding direction will take 128 heart beats to complete. The sequence repetition time (TR) is then the cardiac R--R interval time.
In gated spin echo, data for each slice location is acquired at a fixed delay from the cardiac R-wave. With variations in the cardiac rhythm, the heart may be at a different phase of the cardiac cycle when data is acquired even though the cardiac delay time may be the same. Normal variations of the cardiac cycle usually result in disproportionately larger changes in the diastolic portion of the cardiac cycle, and gated spin echo images acquired at the end of the cardiac cycle often exhibit blurring or ghosting artifacts.
Another disadvantage of gated spin echo is that images at different slice locations are acquired at different cardiac phases. Hence, it may be difficult to relate information from one spatial location to the next as the heart is pictured at different phases of the cardiac cycle. Furthermore, small structures may also be missed due to inadequate temporal and spatial coverage. Motion of the heart during the cardiac cycle can also lead to image contrast variations from slice to slice due to differential saturation or inter-slice cross-talk.
A short TR gated gradient echo pulse sequence may be used to acquire (CINE) images at multiple time frames of the cardiac cycle. As described in U.S. Pat. No. 4,710,717, conventional CINE pulse sequences run asynchronously to the cardiac cycle with the phase encoding value stepped to a new value at each R-wave trigger. In CINE, each rf excitation pulse is applied at the same spatial location and repeated at intervals of TR in the cardiac cycle. Since the sequence runs asynchronously, the rf excitation pulses may occur at varying time delays from the R-wave from one cardiac cycle to the next. On detection of the next cardiac R-wave, the acquired data from the previous R--R interval are resorted and interpolated into evenly distributed time frames within the cardiac cycle. This method of gating is also known as retrospective gating as the data for the previous R--R interval is resorted only after the current R-wave trigger is detected.
The cardiac cycle is divided into equal time points or frames at which images of the heart are to be reconstructed. In order to reconstruct images at each of these time points, data acquired asynchronously is linearly interpolated to the pre-determined time points in the cardiac cycle. In order to account for variations in the cardiac R--R interval during the scan (from changing heart rate), the interpolation varies from cardiac cycle to cardiac cycle, depending on the R--R interval time. This method allows reconstruction of images at any phase of the cardiac cycle, independent of variations in heart rate. As in gated spin echo, only one k-space phase encoding view is acquired per heart beat. The total image acquisition time is then on the order of 128 heart beats.
Faster scan times can be achieved by segmenting k-space and acquiring multiple phase encoding k-space views per R--R interval. The scan time is speeded up by a factor equal to that of the number of k-space views acquired per image per R--R interval. In this manner, a typical CINE acquisition with a matrix size of 128 pixels in the phase encoding direction can be completed in as little as 16 heart beats, when 8 k-space views per segment are acquired.
Multiple phases of the cardiac cycle can be visualized by repeated acquisition of the same k-space segment within each R--R interval but assigning the data acquired at different time points in the cardiac cycle to different cardiac phases. Thus, the cardiac cycle is sampled with a temporal resolution equal to the time needed to acquire data for a single segment, such that EQU temporal resolution=vps.times.TR,
where vps is the number of k-space lines per segment, the TR is the pulse sequence repetition time. The total scan time is then ##EQU1## where yres is the number of phase encoding views in the image. Typically, an image utilizes 128 or more phase encoding views, and 8 views per segment is also often used.
In segmented k-space scans, the total scan time can be substantially reduced by increasing the number of views per segment (vps). However, this is at the expense of reducing the image temporal resolution. As described in U.S. Pat. No. 5,377,680 the image temporal resolution can be increased by sharing views between adjacent time segments to generate images averaged over different time points. The true image temporal resolution is unchanged but the effective temporal resolution is now doubled. View sharing can thus increase the number of cardiac phase images reconstructed without affecting the manner in which the k-space data is acquired.
Prospectively gated, segmented k-space sequences have become popular for cardiac imaging mainly because images can be obtained in a breath-hold and therefore do not suffer from respiratory artifact. Images are formed by acquiring data over a series of heartbeats with data acquisition gated to the QRS complex of the ECG. For images to reconstruct properly, using current methods, the duration of image acquisition must be less than or equal to the duration of the shortest expected R--R interval. In practice, this usually means that the last 10-20% of diastole (.about.100-200 msec for a heart rate of 60 bpm) is not acquired.
Another problem with many current cardiac-gated sequences is that they sort data based on the time elapsed since the QRS complex. As described in U.S. Pat. No. 4,710,787, this strategy assumes that cardiac phase is directly proportional to time. However, in practice the relationship between cardiac phase and the time elapsed since the QRS is not strictly linear. For example, consider sinus arrhythmia where there is a normal, physiologic change in heart rate that accompanies respiration. The time between the QRS complex and end-diastole is longer for those heart beats with longer R--R intervals and in this case, end-diastole is better defined relative to the following (rather than the preceding) QRS complex. This can be seen readily in the normal ECG where the P wave (which signifies atrial contraction) is better correlated temporally to the following (rather than the preceding) QRS complex. This variation in the R--R interval and the fact that a specific cardiac phase occurs at a different delay time from the R-wave with this variation, leads to image blurring in fast segmented k-space pulse sequences and also in conventional CINE pulse sequences.